Phase Angle Of Entrainment Research Articles

Chapter 13 Regulation and integration in the mammalian circadian system

This chapter focuses on some evidence for signal integration within the circadian system at anatomical, cellular, and chemical levels of organization. It also examines the idea that a primary function of the temporal organization of photic and non-photic responsiveness is to precisely define the transitions among organisms' physiological day and night. The influence of non-photic events may be critically important for the day-to-day regulation of physiological and behavioral timing. Phase angle of entrainment is a primary phenotypic expression of the circadian system that is subject to natural selection, and numerous mechanisms may have evolved to adjust species-specific entrainment or to allow individuals to briefly adjust for the daily exigencies of their environment. The fact that circadian systems are designed to allow the interaction of photic and non-photic responses at many levels of organization attests to the need for integration in the regulation of entrainment. The accuracy and stability of entrainment, and the precision of some overt circadian rhythms, rely not only on the ability to respond to a Zeitgeber but also on the ability to integrate the variety of signals that may directly alter the phase or affect the responsiveness of the clock to other Zeitgeber and regulatory input.

Estrus- and steroid-induced changes in circadian rhythms in a diurnal rodent, Octodon degus

Diurnal Octodon degus exhibited marked alterations in activity and temperature in conjunction with the 3 wk estrous cycle when housed in LD12:12 light cycle. On the day of estrus, mean daily activity increases 109%, mean core temperature rises .4°C, activity onset is advanced 2 h, and amplitudes of both rhythms decline compared with the 3 days prior to estrus. On the day following estrus, activity onset was delayed 4.9 h, and mean activity and core temperature fell below that of the preestrus period. Ovariectomy significantly reduced mean temperature (.98°C) but did not significantly alter mean activity, and eliminated cyclic effects of estrus. Estrogen replacement led to a nonsignificant elevation in mean activity and core temperature with no change in the phase angle of entrainment. Progesterone replacement significantly reduced mean core temperature and mean activity, while only the phase angle difference between temperature minimum and activity onset was significantly altered. Intact degus maintained in constant darkness displayed only transient fluctuations in activity onset and temperature minimum during and after estrus. Estrogen or progesterone treatment of ovariectomized, free-running degus altered mean temperature and activity levels, but did not influence tau. Changes in phase angle of entrainment during estrus are not the result of hormone effects on the circadian clock but likely reflect increased or decreased levels of activity.

Effects of age on the circadian system

While aging has been associated with changes in the period and amplitude of circadian rhythms, little is known about how aging influences the response of the circadian clock to environmental stimuli. In this paper, we report on recent studies designed to determine the effects of advanced age on the response of the circadian clock to both photic and nonphotic stimuli in old hamsters (e.g., over 16 mo of age). Among the most pronounced age-related changes in the circadian rhythm of locomotor activity are: (a) alterations in the phase-angle of entrainment to the light-dark cycle; (b) an increase in the magnitude of phase shifts induced by pulses of light presented at specific circadian times; and (c) a loss of responsiveness to the phase shifting or entraining effects of stimuli which induce an acute increase of activity. Depletion of brain monoamine levels in young animals can induce changes in the responsiveness of the circadian clock to environmental stimuli which are similar to those which occur spontaneously in old animals, suggesting that aging alters monoaminergic inputs to the clock. Some of the age-related changes in the response of the clock to an activity-inducing stimulus can be reversed by implanting old animals with fetal SCN tissue. Determining the physiological basis for age related changes in the responsiveness of the clock to both internal and external stimuli, and the mechanisms by which normal circadian function can be restored, should lead to new insight into the functioning of the circadian clock and may lead to new approaches for normalizing disturbed circadian rhythms.

Phase angle changes of photically entrained circadian rhythms following a single nonphotic stimulus

Syrian hamsters entrained to a light-dark (LD) cycle of 14:10 h were given the opportunity to run in novel wheels for 3 h in the middle of the light phase. This manipulation transiently altered the phase angle of entrainment to the LD cycle: activity onset was significantly advanced (by about 0.5 h) on the day after the pulse and gradually drifted back toward its prepulse time. When animals were held in LD 11.5:12.5 h, a photoperiod in which onset time occurs later relative to the time of lights-off, they again advanced about 0.5 h in response to the pulse of wheel running, but many animals retained an advanced phase angle for at least 7 days, and some for more than 21 days. Individual changes in phase angle were highly correlated with the prepulse phase angle: the more negative the phase angle, the greater the advance subsequent to the novel wheel pulse. These results show that a single, short-duration, nonphotic manipulation can produce long-lasting alterations in the phase angle of entrainment to a LD cycle.

Photoperiodic Responses Differ among Inbred Strains of Golden Hamsters (Mesocricetus Auratus)1

Inbred strains of golden hamsters differ in both the free-running period of the circadian rhythm of locomotor activity in constant darkness, and in the phase angle of entrainment of activity to a 14L:10D cycle. To determine whether these differences in circadian entrainment affect photoperiodic time measurement, we measured the critical photoperiod for maintaining testicular function as well as the rate of response for four different inbred strains (MHA/SsLak, LSH/SsLak, BIO 1.5, and BIO 87.20) and an outbred stock (Lak:LVG(SYR)) of golden hamsters. Hamsters of each group were maintained for 12 wk under one of five different LD cycles. Animals of all groups maintained testis size in 14L:10D and 12.5L:11.5D. Significant strain differences were observed in the critical photoperiod for maintaining testis size after 12 wk; the LSH/SsLak inbred strain showed complete testicular regression during exposure to 12L:12D, while little change was observed in any of the other strains under this photoperiod. Some degree of testicular regression was observed in all groups exposed to 11.5L:12.5D, while complete regression was observed in all animals exposed to 6L:18D. The rate of testicular regression differed markedly between the different groups under both 6L:18D and 11.5L:12.5D. The differences in critical photoperiod and rate of testicular regression observed between the various strains could not be correlated with the known strain differences in entrainment or circadian period, indicating that genetic differences in photoperiodic response are not related to the genetic differences in circadian rhythmicity.(ABSTRACT TRUNCATED AT 250 WORDS)

Melatonin injections affect circadian behavior and SCN neurophysiology in Djungarian hamsters

We investigated the effects of daily melatonin (MEL) injection on phase angle of entrainment, duration of wheel-running activity (alpha), and frequency of suprachiasmatic nuclei (SCN) neuronal discharge in the photo-nonresponsive phenotype of the Djungarian hamster, Phodopus sungorus. Photo-nonresponsiveness is characterized by an absence of physiological adjustments to short days (SD). With respect to wheel-running activity, photo-nonresponsive hamsters have a large negative phase angle of entrainment and a compressed alpha under SD. These hamsters also have a delayed nocturnal MEL pulse. These circadian differences are correlated with the daily profile of SCN neuronal activity. In the present experiments, daily MEL injections to photo-nonresponsive hamsters resulted in molt, gonadal regression, and expansion in alpha until entrainment to lights off. Vehicle-injected controls did not exhibit any of these responses. SCN neuronal activity patterns recorded from MEL-injected photo-nonresponders, but not vehicle-injected controls, resembled electrical activity profiles of photoresponsive hamsters. These results demonstrate that MEL induces "photoresponsiveness" in previously photo-nonresponsive hamsters, that MEL modifies circadian behavior to resemble that of photoresponders, and that MEL injections affect the circadian rhythm of SCN neuronal firing.

Effects of aging on entrainment and rate of resynchronization of circadian locomotor activity.

The phase angle of entrainment of the circadian rhythm of the locomotor activity rhythm to a light-dark (LD) cycle was examined in young (2-5 mo old) and middle-aged (13-16 mo old) hamsters. An age-related phase advance in the onset of locomotor activity relative to lights off was seen during stable entrainment to a 14:10-h LD cycle. In addition, the effects of age on the rate of reentrainment of the circadian rhythm of locomotor activity were examined by subjecting young and middle-aged hamsters to either an 8-h advance or delay shift of the LD cycle. Middle-aged hamsters resynchronized more rapidly after a phase advance of the LD cycle than did young hamsters, whereas young hamsters were able to phase delay more rapidly than middle-aged hamsters. The age-related phase advance of activity onset under entrained conditions, and the alteration of responses in middle-aged hamsters reentraining to a phase-shifted LD cycle, may be due to the shortening of the free-running period of the circadian rhythm of locomotor activity with advancing age that has previously been observed in this species.

Circadian characteristics of Djungarian hamsters: effects of photoperiodic pretreatment and artificial selection

Bidirectional artificial selection for (High Line) and against (Low Line) photoresponsiveness altered the percent of photoresponsive hamsters within lines and affected circadian function of hamsters identical in photoresponsiveness. For example, free-running period was shorter in responsive relative to nonresponsive hamsters. Between-line differences for responders and nonresponders were also found: hamsters from the High Line had a shorter free-running period relative to Low Line hamsters. However, phase angle of entrainment to long and short days was not affected. In general, expression of circadian rhythmicity was extraordinarily inflexible in photononresponsive hamsters from both lines: 1) phase angle of entrainment to lights on was similar under short and long day; 2) activity duration was similar under long and short days, although some decompression occurred in constant dark; 3) aftereffects on the free-running period were absent; and 4) amplitude of the phase-response curve was small (+/- 1 h) and present only at circadian times 10-24. We propose that selection for or against photoresponsiveness may have affected the interaction of component oscillators underlying circadian rhythmicity.

Destruction of the hamster serotonergic system by 5,7-DHT: effects on circadian rhythm phase, entrainment and response to triazolam

The role of the serotonergic system in the regulation of hamster circadian rhythms was analyzed using intraventricular injection of the selective neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT). Sixty days after 5,7-DHT administration, immunoreactive serotonin in the forebrain, particularly the suprachiasmatic nuclei and intergeniculate leaflets, was severely depleted in 16 animals, moderately depleted in four and only slightly affected in four. 5,7-DHT produced an immediate and sustained advance of the onset of running wheel activity relative to the 24 h light-dark (LD) cycle. Activity onset occurred0.7 ± 0.07 h before lights out among, 5,7-DHT-treated animals compared with0.18 ± 0.04 h after lights out for vehicle-infused controls. This new, advanced phase angle of entrainment was maintained throughout the 60-day period of the study while the animals remained in a LD cycle, including after an 8-h phase advance of the light cycle. 5,7-DHT treatment also delayed the offset of wheelrunning in 16 of 24 animals and reduced the likelihood of a smooth pattern of reentrainment to the shifted LD cycle. The drug treatment did not affect circadian period in constant darkness, the rate of reentrainment to an 8-h phase advance or the amount of wheelrunning activity per day. In addition, 5,7-DHT treatment had no effect on the ability of triazolam, a short-acting benzodiazepine, to accelerate the rate of reentrainment to an 8-h phase advance. These observations show that ascending projections of midbrain raphe serotonin neurons participate in the regulation of the circadian activity phase but are not required for triazolam-induced acceleration of reentrainment to a phase-advanced LD cycle.

Temperature Dependence of Circadian Rhythms in Golden-Mantled Ground Squirrels

We tested the hypothesis that the free-running period and phase angles of entrainment of circadian locomotor activity rhythms of golden-mantled ground squirrels (Spermophilus lateralis) vary as a function of body temperature (Tb). Animals were maintained in a light-dark cycle (LD 14:10), and at 6-week intervals ambient temperature (Ta) was changed from 20 degrees to 30 degrees C, or vice versa. Data were collected during the animals' homeothermic and heterothermic phases. Subsequently, squirrels were housed in dim constant illumination with the same alternating temperature sequence. In heterothermic ground squirrels (those capable of Tb less than 34 degrees C), a decrease in Ta from 30 degrees to 20 degrees C caused phase delays in activity onset, phase advances in activity termination, and a decrease in duration of the active phase; increases in Ta from 20 degrees to 30 degrees C produced the opposite effect on each of these parameters. The free-running period of the activity rhythm of heterothermic squirrels increased and decreased in response to 10 degrees C decreases and increases in Ta, respectively. Changes in Ta did not affect circadian parameters in homeothermic squirrels. Seasonal variations in circadian organization previously observed in this species appear to reflect the influence of endogenous fluctuations in body temperature. Tissue temperature has a major influence on circadian rhythms of golden-mantled ground squirrels.

Entrainment of anticipatory activity to various durations of food access

Rats with lesions of the suprachiasmatic nuclei (SCN) and intact rats were maintained on restricted feeding with the duration of food access ranging from 4 to 12 hr. All rats with SCN lesions displayed at least some anticipatory activity (AA) at all food access durations. The amount of AA diminished when food access was extended to 12 hr and was lowest in a group that was exposed only to a 10-hr access period. The onset of AA (phase angle of entrainment) appeared to be more sensitive to the time of food availability than to its termination. Most intact rats maintained in constant light displayed some AA at food access durations between 4 and 10 hr. In most cases the period of the free running rhythm increased as it crossed food access and the free running rhythm became increasingly disrupted as the experiment progressed in all rats. In some cases the free running rhythm appeared to force AA out of entrainment. These results demonstrate that AA occurs in conditions that impose only minor deficits on the energy balance of rats. Furthermore, they provide additional evidence of interactions between two separate circadian pacemaking systems.

Entrainment of the circadian rhythm of wheel-running activity is phase shifted by ablation of the intergeniculate leaflet

The phase angle of entrainment of golden hamster wheel-running activity was determined before and after bilateral destruction of the intergeniculate leaflet (IGL). Animals were maintained under continuous dim light which rhythmically varied in intensity (10-5 lux) in the form of a sine wave with a 24 h period. The phase angle of entrainment changed significantly following IGL ablation suggesting a role for the IGL in entrainment.

Activity rhythms and photoperiodism of syrian hamsters in a simulated burrow system

Male Syrian hamsters were housed in simulated burrows in order to investigate (a) how these nocturnal, fossorial rodents entrain to the prevailing light:dark cycle in this semi-natural habitat and (b) the response of the reproductive system to environmental illumination. The burrow emergence activity of hamsters housed in simulated burrows was compared to the running wheel activity to animals maintained in standard cage conditions. The activity rhythm was similar in both measuring devices. The data suggested that in a natural environment hamsters are only exposed to light for short amounts of time each day. To determine whether brief photoperiodic stimulation could alter the phase angle of entrainment and/or the reproductive condition, burrow housed animals were exposed to a supplemental 30-second light pulse during specified clock hours of the dark period on a daily basis. These light pulses induced a phase shift and maintained a long day reproductive response in what was otherwise a short photoperiod.

Circadian organization and neural mediation of hamster reproductive rhythms

(1) Circadian rhythms mediate various cycles of the hamster's reproductive physiology and behavior. (2) Destruction of the suprachiasmatic nuclei interfered with the integration and/or generation of coherent circadian rhythms of wheel-running behavior and also abolished photoperiodic regulation of the testes, which remain large and functional after the lesion regardless of photoperiod or presence of the eyes. (3) The retinohypothalamic tract is the principal visual projection for the entrainment of circadian activity rhythms to the light: dark cycle and also appears necessary and sufficient for mediating the effects of photoperiod on the reproductive physiology of the male hamster. (4) The hamster estrous cycle free-ran under constant dim illumination with a period significantly different from 4 days; regression analyses of concurrently recorded free-running heat onsets and wheel-running behavior showed that the period of the estrous cycle was a multiple (× 4) of the circadian activity period. This implies a circadian organization of the estrous cycle and mediation of both activity and estrous behaviors by a common circadian system. Implications of these findings for contemporary views of sexual differentiation of gonadotrophin release are considered. (5) Estradiol influenced the phase angle of entrainment of wheel-running behavior by ovariectomized hamsters. The period of the free-running activity rhythm was significantly shortened in blind ovariectomized hamsters implanted with Silastic capsules containing estradiol benzoate. The functional significance of these hormone-mediated changes in rhythm periodicity is discussed. (6) Exposure of male hamsters to short daily photoperiods reduced the frequency of copulation. Castrated males maintained on short daily photoperiods were less responsive than those on long days to the activational effects of testosterone on copulation. The significance of this photoperiodically induced refractoriness to hormones, which in this species appears limited to the male, is discussed.